1. Field of the Invention
This invention relates to sewage sludge solids and the production of fertilizers from them, and more particularly to a new method for converting sewage sludge solids into attrition resistant, controlled release fertilizer agglomerates, or granules, by blending the sludge solids with liquid ureaform, heating, and mechanically compacting the blend.
2. Description of the Prior Art
The disposal of waste products has always been a technical and economic problem. Significant progress has been made in solving this problem by the development of improved sewage treatment methods. The activated sludge process is particularly important, whereby municipal, agricultural, and industrial sewage is converted to water suitable for return to the waterways, and to solids for disposal, by action of bacteria and enzymes. Sewage sludge solids are frequently recovered as dry solids for use as plant nutrients and micronutrients. The dried sewage sludge solids have soil conditioning properties. These dry solids are used as plant foods on some specialty crops such as grass, ornamental plants, and flowers. The low bulk densities, such as 25 pounds per cubic foot, and low nutrient concentrations, such as 6-2-0.5, preclude extensive use as a commercial fertilizer outside the immediate area of its production. The low bulk density also precludes its effective use for bulk blending with other more dense fertilizer materials to form complete fertilizers.
John Karnemaat, in U.S. Pat. No. 3,655,395 disclosed a process for treating industrial and municipal waste materials which are suspended or dissolved in water, by stepwise treatment with concentrated aqueous formaldehyde, nitric acid, and urea to form urea-formaldehyde condensation products. He dried the condensation products thus formed. His process constituted a significant step forward in the art of treating sewage sludge solids. The process undesirably required the use of aqueous sewage sludge solids which usually have an unpleasant odor, and strong aqueous formaldehyde solutions, about which questions have been raised regarding its toxicity and safe handling. Also, nitric acid is required in the process, and it generates nitrogen oxide gas which is toxic. Nitric acid poses a possibility of explosion when it is mixed with organic materials and heated, as in Karnemaat's drying operation.
Karnemaat, in U.S. Pat. No. 3,939,280, discloses an extension of his technology whereby poultry manure is converted to animal feeds by treatment with a catalytic amount of a non-toxic acid, such as sulfuric, phosphoric, citric, propionic and acetic, and a formaldehyde-supplying substance, and urea. He dried the mixture.
O'Donnell, in U.S. Pat. No. 3,942,970, discloses a system for, and a process of, producing a granular, high-nitrogen, odorless fertilizer from waste filter sludge material, which requires reducing the particle size of the sludge and reducing the moisture content of the 1/8 inch, or smaller, sludge particles to between 30 and 50 percent and adding acid to bring the pH of the sludge to between 3.0 and 5.0. O'Donnell prepared an alkaline N-methylol-urea aqueous prepolymer comprising mixed mono- and dimethylol urea by reacting 1.1 to 2.0 mols of urea with a mol of formaldehyde for 10 to 30 minutes at 30.degree. to 80.degree. C. He immediately reacted the aqueous prepolymer with the sludge at a temperature of 30.degree. to 80.degree. C. at a pH of between 3 and 5 with vigorous agitation to provide a granular reaction product of condensed urea formaldehyde polymer having a molecular weight of 300 or less, admixed with the dried sludge material of 3 to 10 percent moisture.
In U.S. Pat. No. 4,081,366 O'Donnell extended his technology to define a process for dewatering a solid reaction material from an aqueous solution containing organic waste material, by reacting in the manner of his previous technology and then adding dewatering and recovery steps. The dewatering and recovery is achieved by means of vacuum filtration.
The term ureaform liquid is used herein to indicate a urea formaldehyde solution having a molar excess of urea over formaldehyde. Those solutions as used in most fertilizer processes, are very reactive, as indicated by their use in O'Donnell's methods, initially containing formaldehyde available for reaction, and are generally not stable enough to store and ship. In U.S. Pat. No. 4,304,588 W. P. Moore disclosed a storage stable, concentrated, uncondensed, ureaform solution for use as a foliar fertilizer. This ureaform was prepared with an alkaline buffering catalyst, and with a carefully controlled pH and amounts of hexamethylene tetramine and/or ammonia to prevent condensation during storage. The term uncondensed, as used herein, indicates that the ureaform components remain substantially as monomeric chemical compounds containing little, or no, polymeric materials, or free formaldehyde.
Although progress has been made in the conversion of sewage sludge solids to fertilizers, the dry sewage solids produced by the methods of the prior art have low bulk densities, and the fertilizer particles produced have little structural integrity. These properties cause the sludge solids to segregate and to break-up, forming fines and dust when handled with commercial fertilizer blending or handling equipment. No method has been provided in the art for converting to dense fertilizers the very low density solids, obtained in such places as cyclone separators and fines screens in existing sewage sludge solids drying plants.
The methods of the prior art for producing low odor, slow release fertilizers from sewage sludge, require the treatment of shredded sludge filter cake with formaldehyde and urea, or freshly prepared mono- and dimethylolurea which must be used immediately after it is formed.
No methods have been disclosed for converting low density sewage sludge solids to attrition resistant, controlled release fertilizer agglomerates.